It is often necessary or desirable to infuse a flowable material, which may be liquid, a gas or a combination thereof, into a patient. One example is the administration of parenteral fluids to a patient.
A typical infusion system includes an infusion device for delivering flowable material and conduit means for conducting the flowable material from the infusion device to the patient. The conduit means typically comprises flexible tubing leading from the infusion device and a needle for insertion into the vascular system of the patient. In normal operation, the infusion device delivers the flowable material through the tubing and the needle to the vascular system of the patient.
One problem with infusion systems of this type occurs when the open distal end of the needle is not in communication with the interior of the vessel into which the flowable material is to be infused. This condition, which is known as infiltration, may occur, for example, when the needle is advanced through the vessel wall into tissue. In this event, the parenteral fluid will not be supplied to the interior of the vessel, and so the flowable material is improperly supplied by the conduit means to the patient. A similar problem may arise when attempting to infuse a flowable material into other regions of the body.
It is known to monitor the pressure of the flowable material delivered to the patient and to use the pressure information thus obtained for various control purposes. Examples of this are shown, by way of example, in Jenkins U.S. Pat. No. 4,277,227, Rodler U.S. Pat. No. 4,457,751 and Nelson U.S. Pat. No. 4,534,756. The system disclosed in the Nelson patent looks at different characteristics of the pressure being monitored depending upon the infusion flow rate to determine infiltration. For example, one characteristic is utilized for low flows and another characteristic is used for higher flows. This tends to complicate the infiltration detection apparatus. In addition, the system disclosed in this patent is limited to use with an infusion device whose normal delivery pattern or output is pulsed.